1. Field of the Invention
The present invention relates to a highly reliable thin-film transistor (TFT) and a fabrication method thereof. The thin-film transistor which is fabricated according to the present invention has the characteristic that it is formed on an insulating surface. In the present invention, an insulating surface does not only mean an insulating surface such as glass, but also includes an insulating covering which is provided on a semiconductor substrate such as amorphous silicon. In particular, the present invention relates to a thin-film transistor which has a process in which the source/drain regions are activated by thermal annealing.
2. Description of the Related Art
Research has been conducted recently on insulating gate-type semiconductor devices which have a thin-film semiconductor layer (also referred to as an active layer or active region) which is provided on an insulating surface. In particular, thin-film insulated gate transistors, so-called thin-film transistors (TFT) are earnestly studied. These are used in the drive circuits of matrix circuits, or for controlling pixels in an image device such as a liquid crystal display device which has a matrix construction formed on an insulating substrate, and they are also used in integrated circuits which have a three-dimensional construction, in which semiconductor integrated circuits are formed in multiple layers. TFTs are classified as amorphous silicon TFTs or crystalline silicon TFTs, depending on the material and crystalline form of the semiconductor being used.
In general the electric field mobility of semiconductors in the amorphous state is low, and they can therefore not be used in TFTs, which require high speed operation. Further, the P-type electric field mobility of amorphous silicon is very low, and therefore P-channel type TFTs (PMOS TFTs) cannot be fabricated, and it is consequently impossible to form complementary MOS circuits (CMOS) by combining with an N-channel type TFT (NMOS TFT).
However, the electric field mobility of crystalline semiconductors is higher than that of amorphous semiconductors, and therefore high speed operation is possible. Furthermore, with crystalline silicon, not only can NMOS TFTs be obtained, but also PMOS TFTs can be obtained in the same way, and therefore CMOS circuits can be formed; for example in an active matrix type liquid crystal display device, not only can the active matrix circuits, but also the peripheral circuits (driver circuit etc.) which drive this can be constructed on one and the same substrate using CMOS crystalline TFTs, and it is possible to obtain a so-called monolithic construction.
Recently, in order to reduce deterioration due to hot carriers, there has been a demand for the provision of low density impurity regions adjacent to the source or the drain regions, in which the density of N-type or P-type impurities is less than in the source/drain regions.